There is no reason it shouldn't be available. I assume the problem is with Steam's paradigm: Almost everything available on Steam is software, which only supports whichever platforms the developer planned for. So when a publisher adds an item to Steam, they're required to tick a "Suppored Platforms" box (i.e. (x) Windows (x) Mac ( ) Linux). There was no Linux beta when Indie Game was published on Steam, so there was no Linux option to checkmark. Now there is, and but they haven't edited it yet.

Kinda old but, well, you can just go to SteamApps/common/Indie Game The Movie/data/movies and try to watch it from there. Not booted in my fedora right now and not really in the mood to restart my computer just to test that, but it should be a normal video file, so, watchable with any video player.